The present invention relates in general to a signal processing apparatus. In particular, the invention concerns a signal processing apparatus which is preferentially suited for encoding an analog video signal into a binary signal, which analog video signal is obtained through photoelectric conversion of an original image by means of a photoelectric converting element array.
In facsimiles (Fax) and optical character reader systems (OCR), the analog video signal produced through the photoelectric conversion is encoded into a binary signal having a "white" level and a "black" level (or a ternary signal having additionaly a "gray" level). When an image of an original to be photoelectrically converted is illuminated by a light source having an elongated bar-like configuration and/or when the image is focussed through an optical lens system onto a light sensitive face of the photoelectric converting element array, there will frequently happen such a phenomenon in which a quantity of light tends to be insufficient at a peripheral portion of the image as compared with a mid portion thereof, involving eventually distortion in the analog video signal produced through the photoelectric conversion. This means that the encoding of such analog video signal into the binary signal with reference to a reference signal voltage of a predetermined magnitude will result in unreliability of the encoded binary information signal. As an attempt to evade such inconveniences, there has been already proposed a floating slice method according to which the reference signal voltage is caused to vary in following up the aforementioned distortions of the analog video signal by integrating the analog video signal to be encoded into the binary signal. However, the floating slice method is disadvantageous in that when a "gray" information continues to exist, the reference signal voltage is subjected to fluctuations in dependence on the characteristics of an integrating circuit as employed and the contents of the "gray" information, resulting in that the "gray" information is undesirably encoded into the "white" level information or alternatively into the "black" level information.
Further, there has been proposed a system in which the magnitude of the output signal from the photoelectric converting element array which corresponds to the "white" level of the original image is previously stored, whereby the analog video signal derived through the photoelectric conversion of the image is subsequently divided by the stored value thereby to correct the distortions of the analog video signal (refer to Japanese Laid-Open Patent Application Publication No. 95518/1978). However, this system naturally requires a rather complicated circuit configuration due to the necessity of a divider circuit for a high frequency on the one hand, while on the other hand, difficulty is encountered in correcting appropriately the analog video signal, when the background density of an original undergoes variations on the way of the scanning thereof.
It has been also proposed that the photoelectric conversion efficiencies of the individual photoelectric converting element constituting the photoelectric converting element array are previously stored, wherein the binary encoding is effected by comparing the analog video signal derived through the scanning and photoelectric conversion of an image with a reference signal voltage produced on the basis of the stored contents mentioned above (refer to Japanese laid-open patent application publication No. 81419/1975). However, this approach is also disadvantageous in that the binary encoding can not be accomplished with any reasonable accuracy because the reference voltage does not vary in following up possible variations in the background density of the original.